The following prior art documents are believed to summarize the state of the art:
U.S. Pat. Nos. 5,982,921; 4,579,455; 4,969,198; 4,805,123; 5,185,812; 6,818,459; 6,816,249; 6,282,309; 6,081.325; 6,078,386; 6,020,957; 5,917,588; 5,825,482; 4,845,558; 4,579,455; and 4,532,650, and published U.S. Patent Application US20040066507A1.